Understanding the Faraday Cage
What Is the Physics of Shields for Electromagnetic Fields and Waves?
A Faraday cage is an enclosed conducting shell that shields its interior from strong electric fields and electromagnetic waves.
Charges and Electric Fields in Conductors
As a consequence of Gauss's Law, all the excess charges, positive or negative, in an electrical conductor must reside on the surface of the conductor, if the conductor is in equilibrium. Furthermore the electric field inside the conductor will be zero. Charges are free to move around in a conductor in response to any electric forces or fields, so the charges will rearrange themselves until the electric field inside is zero.
Faraday Cage Effect
The Faraday cage, which is an effect discovered by Michael Faraday in 1836, depends on these principles. A Faraday cage is an enclosed conducting shell. It can be a solid conducting shell or a shell made of wire mesh; the important thing is that it completely encloses a region.
This hollow conducting shell will have no electric field inside, even when placed in a very strong external electric field because the charges on the conducting surface rearrange themselves until the electric field inside is zero. A Faraday cage also works in reverse. If there is a strong electric field inside, the field outside the cage will be zero.
This Faraday cage effect causes Faraday cages to act as shields for strong electric fields or other electrical effects. In addition electromagnetic waves consist of oscillating electric and magnetic fields. Therefore Faraday cages effectively shield electromagnetic waves or electromagnetic radiation as long as the holes in the wire mesh are significantly smaller than the wavelength of the electromagnetic waves. For this reason Faraday cages are sometimes called Faraday shields.
Applications of Faraday Cages
Perhaps the most dramatic application of Faraday cages is lightning safety. If lightning strikes a closed metal aircraft or car, the occupants are safe as long as they are not in electrical contact with the outside metallic surface. The enclosed metallic car or aircraft acts as a Faraday cage and shields the interior from the strong electric field of the lightning strike.
AC currents induce changing magnetic fields which in turn induce electric fields. These electric fields can disrupt sensitive electronic devices, so they use Faraday cages to shield crucial electronic components from stray electromagnetic fields. Coaxial cables are surrounded by a conducting shell to provide Faraday shields.
Faraday cages block electromagnetic waves. Buildings or rooms can be deliberately built as Faraday cages either to prevent electromagnetic interference for sensitive electronics or to prevent external spying in high security situations. Buildings or rooms with significant amounts of metal in their construction might not be deliberate Faraday cages. They can however act as such and block cell phone and other wireless communication.
The metal shell of a microwave oven also acts as a Faraday cage and traps the microwaves inside the oven.
In addition to Faraday cages, Michael Faraday discovered Faraday's law for electromagnetic induction.
Further Reading
Hecht, E., Physics:Algebra/Trig, Brooks/Cole, 1997.
Knight, R.D., Physics for Scientists and Engineers with Modern Physics, Pearson, 2004.
Wilson, J.D., Buffa, A.J., and Lou, B., College Physics 6th ed., Pearson, 2007.
